JCECE Engineering JCECE Engineering Solved Paper-2012

  • question_answer
    If\[\mathbf{a}=2\mathbf{i}+2\mathbf{j}+3\mathbf{k}\],\[\mathbf{b}=-\mathbf{i}+2\mathbf{j}+\mathbf{k}\]and\[\mathbf{c}=3\mathbf{i}+\mathbf{j}\], then \[\mathbf{a}+t\mathbf{b}\] is perpendicular to \[\mathbf{c};\] if\[t\] is equal to

    A) \[2\]                                     

    B) \[4\]

    C) \[6\]                                     

    D) \[8\]

    Correct Answer: D

    Solution :

    \[\mathbf{a}+t\mathbf{b}\bot \mathbf{c}\] \[\Rightarrow \]               \[(\mathbf{a}+t\mathbf{b})\cdot \mathbf{c}=0\] \[\Rightarrow \]               \[\mathbf{a}\cdot \mathbf{c}+t\mathbf{b}\cdot \mathbf{c}=0\] \[\Rightarrow \]               \[t=-\frac{\mathbf{a}\cdot \mathbf{c}}{\mathbf{b}\cdot \mathbf{c}}\] \[\Rightarrow \]               \[t=\frac{(2\mathbf{i}+2\mathbf{j}+3\mathbf{k})\cdot (3\mathbf{i}+\mathbf{j})}{(-\mathbf{i}+2\mathbf{j}+\mathbf{k})\cdot (3\mathbf{i}+\mathbf{j})}\] \[\Rightarrow \]               \[t=\frac{6+2+0}{-3+2+0}=8\]


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